FILE - Relatives and friends hold banners with images of some of the 43 missing students of the Rural Normal School of Ayotzinapa as they march in Mexico City to mark the first anniversary of the students' disappearance, Sept. 26, 2015.

The former head of investigations for the Mexican attorney general's office, who has been criticized for his handling of the missing-persons case involving 43 students, was named a national security adviser hours after he resigned his previous post.

A statement from the Interior Department late Wednesday announced that Tomas Zeron would be technical secretary for the National Security Council, a position appointed by and reporting directly to President Enrique Pena Nieto.

Zeron's dismissal had been demanded by the families of the 43 teachers college students who disappeared in September 2014, after they were taken by local police in Iguala, in Guerrero state, and have not been heard from since. They were allegedly handed over to a drug gang and slain.

Zeron was at the center of the government's widely criticized investigation. Two independent teams of experts have cast doubt on its insistence that their bodies were incinerated at a trash dump.

FILE - Tomas Zeron, former head of investigations for the Mexican attorney general's office, speaks during a news conference in Mexico City, Jan. 13, 2015.

Zeron oversaw the criminal investigation agency of the attorney general's office and also its forensic work.

Human rights activist Mario Patron said in an interview with Radio Formula on Thursday that Zeron's departure and subsequent appointment to the National Security Council appeared to signal that he would be absolved of errors in the investigation of the disappearances.

At a news conference Thursday in the Mexican capital, Mario Gonzalez, the father of one of the students who vanished, said students' families continue to be frustrated with the investigation and suggested that officials had withheld evidence to protect unnamed persons.

"For us it is outrageous that when he has an open investigation, they reward him with a higher position,'' Gonzalez said. "It is a mockery, not only for the parents of the 43 but for all Mexicans.''

The Interior Department statement said Zeron's appointment was "a recognition of his actions" and reflected "the experience and capability he has demonstrated in his previous positions.''

The missing students attended the Rural Normal School of Ayotzinapa. They were in Iguala on September 26, 2014, and were en route to a rally in Mexico City when police stopped them and allegedly handed them over to members of the Guerreros Unidos cartel.